1. Field of the Invention
The present invention relates to a cooling structure for an electronic device which is equipped with heat generating elements, and more particularly, to the structure of a tank for keeping a coolant liquid (hereinafter simply called the “reservoir tank”) in a circulation-type liquid cooling system.
2. Description of the Related Art
In recent years, as the performance of electronic devices increases, parts mounted in such electronic devices have generated increasingly more amounts of heat. As a result, stricter requirements have been imposed on cooling technology for devices. In the field of computers, as a result of increasing the number of transistors on a processor (CPU) and raising the operating clock frequency of the CPU to improve the processing performance and processing speed of the CPU, electric power density of a chip has increased to such a degree that TDP (Thermal Design Power) exceeds 30 W even in mobile applications. As such, there is an urgent necessity to establish an effective cooling technology for removing heat generated within a housing.
Conventionally, for cooling an electronic device such as a personal computer, a heat sink is connected to a heat generating element such as a CPU to spread heat which is then discharged to the outside of a housing by use of forced air cooling. Recently, however, device-cooling methods based on liquid cooling technology have been under investigation because of their higher heat radiation performance and quiet operation.
FIG. 1 illustrates the structure of a liquid cooling system mainly used in personal computers.
Liquid cooling system 1 comprises radiator 3a, circulating pump 4a, reservoir tank 5a, and heat receiving element 2a such as a cold plate. Heat receiving element 2a is connected to heat generating element 6 such as a CPU and a GPU for absorbing heat therefrom, and transports the amount of received heat to radiator 3a through a coolant liquid which flows within heat receiving element 2a. Radiator 3a exchanges heat with external air through natural air cooling or a combination of the natural air cooling with forced air cooling in order to radiate heat. The coolant liquid cooled by radiator 3a is transported again to heat receiving element 2a by circulating pump 4a. The circulation-type liquid cooling system configured in the manner described above is provided with reservoir tank 5a for keeping an amount of coolant liquid that is required to compensate for a lost amount of the coolant liquid due to volatilization from component members (mainly from resin tube 7a).
FIGS. 2 and 3 illustrate a modular structure of such a liquid cooling system. FIG. 2 illustrates a general-purpose liquid cooling module for use in desk top personal computers (PCs) and the like, while FIG. 3 illustrates a thin-type liquid cooling module for use in notebook-type personal computers and the like. While both modules employ common components, the shape of a radiator and the like may be modified in accordance with the characteristics of a particular housing in which the liquid cooling system is mounted.
The aforementioned reservoir tank, which forms part of a liquid cooling system, is responsible for the following three functions: (1) keeping a required amount of coolant liquid during a device for a guaranteed period; (2) alleviating of variations in the pressure within a circulating system due to an expanded volume of the coolant liquid caused by received heat; and (3) trapping and removal of bubbles generated within the circulating system.
A resin tube for interconnecting components of a liquid cooling system permits volatilization of moisture of the coolant liquid through its molecular interstices. For this reason, anti-freeze condenses over a long-term use, and variations in viscosity due to a change in concentration degrades the coolant circulating performance, resulting in lower cooling performance of the system. Therefore, for guarantee the operation for a specific period, it is necessary to keep a surplus amount of coolant liquid in anticipation that an amount of moisture will volatilize, so that a reservoir tank is provided for pooling the coolant liquid.
Also, during the operation of an apparatus, coolant liquid, which has absorbed heat from heated devices, expands in volume as it is heated to increase the inner pressure of a circulating path. In this event, an air layer must be provided, as well, for alleviating the pressure within the circulating system in order to prevent the coolant liquid from leaking from a module junction. To meet this requirement, the liquid level (boundary between the liquid and air layer) is adjusted within the reservoir tank to ensure that there is a required amount of air capacity.
Likewise, during operation of the apparatus, bubbles can be produced within the circulating path due to introduction of air from the outside of the system, cavitation, decomposition of the liquid, and the like. If the bubbles stay within the path and block it, the coolant liquid cannot be circulated, possibly causing a loss in the cooling capabilities of the system. Also, if the bubbles within the liquid enter a thin flow between a rotor and a main shaft of a circulating pump, a gap is blocked which results in a semi-dry lubrication or a solid lubrication state between the main shaft and the rotor, which causes the generation of sudden abrasive heating which would damage the pump's bearings. Consequently, the lifetime of the pump is reduced. Further, if the bubbles are deposited such that they cause the entire rotor to be surrounded by an air layer, the pump becomes incapable of pumping the coolant liquid, resulting in a failure to circulate the coolant liquid. For this reason, bubbles staying within the system are released into the air layer to alleviate the pressure within the reservoir tank.
Exemplary applications of such a liquid cooling system are known from configurations described in JP-A-266474/1994, JP-A-366260/2002, JP-A-022148/2003, JP-A-209210/2003, JP-A-047842/2004, and the like. FIGS. 4A and 4B illustrate an example of a conventional liquid cooling system which is disclosed in JP-A-209210/2003. Heat receiving element 2d such as a cold plate is mounted on the main body of notebook type personal computer 10 to cool down CPU 11, while a thin radiator comprised of radiator tube 13 and metal radiator plate 14 are mounted on display 12 to dispel heat.
In this configuration, reservoir tank 5d that is used to store coolant liquid is fixed on the radiator to maintain a required amount of coolant liquid for a guaranteed period and to remove bubbles produced in the circulating system, thereby helping the circulating pump to operate normally.
When such a liquid cooling system is employed as a cooling means for an electronic device that can be installed in different orientations, depending on the requirements of a particular user, the function of the aforementioned reservoir tank does not work well in some situations.
Taking as an example a projector apparatus for projecting an image onto a screen, the apparatus may be installed on a floor for use as illustrated in FIG. 5A, or the apparatus may be suspended from a ceiling for use in an inverted position as illustrated in FIG. 5B. In addition, depending on certain conditions, it is contemplated that the apparatus may be installed in an upright posture for projection through reflection at right angles.
However, the conventional liquid cooling system is designed on the assumption that the apparatus with which it is used operates in the same orientation at all times. Also, the reservoir tank is not designed to support any orientation in an apparatus in which the reservoir tank is installed at any angle over 360 degrees, rather, the reservoir tank is simply a closed case having an inlet port and an outlet port with a proper amount of coolant liquid 24a stored therein.
The conventional reservoir tank in such a structure is highly likely to significantly lose cooling performance due to an air layer within the tank which readily blocks the conduit inlet and outlet ports, following a change in the position of the apparatus, which prevents the coolant liquid from circulating. In particular, in the reservoir tanks disclosed in JP-A-304086/2003 and JP-A-08958/2004, only the opening at the end of an outlet tube within the tank is positioned at the center of the tank, while the opening at the end of an inlet tube within the tank is set largely spaced away from the opening at the end of the outlet tube within the tank. In this structure, the reservoir tank does function without an air within the tank approaching to the opening at the end of the inlet tube within the tank as long as the position is changed within a particular range (from 0° to 90°), but the reservoir tank is not designed to accommodate any changes in positioning from 0° to 360°.
JP-A-078271/2003 describes an exemplary reservoir tank which is capable of supporting a change in orientation from 0° to 360°. However, the disclosed reservoir tank has the open end of an inlet tube and the open end of an outlet tube arranged in parallel in the same direction at the center of the tank. In this arrangement, since the open ends of the inlet tube and outlet tube are exposed, a change in the orientation of the tank is likely to cause air in the reservoir tank to flow again into a conduit. In other words, even if bubbles in the path are trapped and kept in the air layer within the tank, the bubbles will flow back into the conduit when the orientation of the tank is changed, resulting in a high susceptibility to detrimental effects such as clogging of the conduit by bubbles, damage to a pump, and the like.